Commercially available gas lasers offer limited power output capabilities per unit length. Typically, the upper limit for a CO2 laser is about 0.6 watts/cm. Hence, when a CO2 laser is used in applications requiring high output powers (about 30 watts), resonator lengths are on the order of 50 cm. The resulting system architectures are mechanically complex, are not rugged or reliable, and have restricted portability. When waveguide lasers are used with a ceramic bore material, long resonator lengths imply considerable manufacturing difficulties because of tight machine tolerances, poor yields, and high production costs.
Chenausky et al, in U.S. Pat. Nos. 4,429,398 and 4,438,514, solves the length problem by using mirrors to fold the optical path, achieving some reduction in overall resonator length at the expense of a modest increase in width. In the '398 patent two parallel waveguides are optically coupled by a pair of folding mirrors within a common block. This configuration results in a finite amount of diffraction losses that can be reduced but not eliminated by minimizing the distance traveled in free space. The '514 patent shows a progressive improvement which uses a third diagonal waveguide that connects, end-to-opposite end, the two parallel waveguides. Since the waveguides are serially connected, the laser cavity will have a series of parallel waveguides, a Z-folded waveguide, or a ring arrangement.
De Maria, in U.S. Pat. No. 3,361,987, increases active gain length in a polysided folded laser path in resonators employing solid or liquid amplifier media, with a continuously folded optical path, using total internal reflection. Nakagome et al in U.K. Application No. 2,033,648A employs a similar configuration for achieving long optical paths in semiconductor light amplifiers. The use of total internal reflection by De Maria and Nakagome requires that the refractive indices of the laser medium and the matching medium be selected to achieve the proper angle of reflection. The reflecting medium may surround the laser medium, or may be deposited on the end faces of a solid medium. De Maria suggests the use of a ruby laser element immersed in liquid oxygen. The required index matching significantly increases the difficulty of manufacturing and operating the laser. In gas lasers with the index of refraction differing from unity by a few parts per million, total internal reflection cannot be used as an optical path folding technique.
Reduction in resonator lengths can be achieved by allowing resonator cross-sections to grow proportionately to maintain a constant volume of lasing medium. However, where the laser circuitry cross-section cannot grow past an upper limiting size, such as the capillary bore in HeNe lasers or small channels in waveguide CO2 lasers, the standard technique to reduce length has been to divide the laser resonator into smaller segments and to connect the segments serially with mirrors or prisms.
Successful implementation of laser-based systems can greatly benefit from a drastic reduction in laser resonator size. In particular, overall laser cavity length appears to be the limiting parameter. Virtually all applications that require portability and ruggedness are enhanced by a reduction in total cavity length.